selftests/resctrl: Remove unnecessary startptr global from fill_buf

fill_buf stores buffer pointer into global variable startptr that is
only used in fill_cache().

Remove startptr as global variable, the local variable in fill_cache()
is enough to keep the pointer.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Tested-by: Shaopeng Tan (Fujitsu) <tan.shaopeng@fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>